Slightly Embarrassing Question

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dartman59

dartman59

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1971 Dart. 408 (360 LA block). I’ve been chasing what I think is a drivetrain vibration for a couple years (!). Lately, it occurred to me that maybe it’s an engine balance thing. I don’t think it is... the vibration is in the seat of my pants, starts being noticeable at 30 mph and gets steadily worse the faster the car moves. At 80 it’s shaking pretty bad. Also, at 80, if I mash in the clutch the vibration changes little if at all. However, in that condition, if I rev the engine to, say, 3000 rpm, I can definitely feel a “lighter” vibration, that I associate strictly with the motor revolutions. Four speed; Detroit locker diff, 3.55, centerforce clutch. Thoughts? If my engine balance was off, I would definitely know that, right? I don’t know if my configuration is external or internal balances, but my builder (I won’t mention any names yet because I think there’s no problem there) says it’s internal balance and a neutral h balancer is what I need, and that’s what’s on there currently. Any and all responses appreciated.
 
What flywheel did you use with it?
Did your machinist balance everything as one unit, balancer, flywheel etc?
I would take the driveshaft out and have it balanced, before I tore anything down.
 
The first thing that I would do is rule out the trans, driveshaft and the rear...

What happens if you rev gradually up the rpm range in neutral/parked?

What happens if you go to 30mph+ and shut the motor off? (Be careful trying that one)

Also when is the last time you changed your cap/rotor/plug wires? It could be a misfire under load...
 
My Centerforce vibrates too,lol.
The stinking flyweights seem to hang up, Whenever I hit the hiway, and once at cruising speed, I have to declutch an blip the throttle sometimes two or three times. I cruise at 65=2240
 
AJ/FormS said:
My Centerforce vibrates too,lol.
The stinking flyweights seem to hang up, Whenever I hit the hiway, and once at cruising speed, I have to declutch an blip the throttle sometimes two or three times. I cruise at 65=2240
Click to expand...
There a little vibration in my motor when standing still and revving, but not much. I haven’t actually shut the motor off at 30 mph but my sense is not much would change in what I perceive as a driveline vibration.
 
Haven’t done that but probably should. I would be FLABBERGASTED to find out that was the cause. It feels very mechanical and very much tied to vehicle speed. Also, when I let off the gas the vibration gets worse, like it’s my driveshaft flopping around. I guess my question is “could I have driven for 15000 miles with the wrong harmonic balancer or would it have been way crazy if I was using an internally-balance balancer if I was supposed to be running an external balance harmonic balancer.
 
your builder could have neutral balanced your 360. But you would need a neutral flex, balancer and convertor too.
 
what tires are you running, i just put a set on a 2008 dodge pickup and got to having a real bad vibration, i though it was a tire out of round,got to reading the sidewall markings, even thought they were radials they were made of nylon, and everybody from the 70s remember how they got flat spots when they sat, after driving about 40 mile the flat spots would start rounding out. I thought they had done away with nylon tires, but i assure they haven't, had mine put on in dec,2017, already took them off and sold them and went to polyester to get rid of the vibration...just a thought
 
I agreed with SSG because the 408/4-speed combo will whack the front spring segment around pretty good with street type or OEM springs,with power on and power off. The pinion angle will be flipping 3 to 5 or even 8 degrees with a 4 speed,
 
My Dart had a vibration that I chased for years. Finally this spring while the transmission was out I took the driveshaft to a local shop to have it checked. My $450 Dynotech shaft was out of balance, $35 and it's now smooth as silk.
 
Thanks everyone. SSG-yeah that was my initial thought. Two years ago I got that sorted out. It took a couple of shims but it’s right on the money now. Doing that changed the character of the vibration but didn’t eliminate it by any means. Justinp61–you must have been ecstatic when that happened. I put this car together three years ago and it’s all pretty new—crate 408, rebuilt 833 by Brewers, new 7290 slip yoke and driveshaft by Victory, new Detroit locker (489), new 3.55 gears, new axles, new wheels, new tires. I would love to have this resolved!
 
Driveshaft.
Cheapest thing to start with.
Have it checked by a reputable shop.
I had a few shortened, its suprising that the yokes arent dead square.
 
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Just out of curiosity - did you dial indicate the bellhousing or verify the engine and trans is sitting in the car 100% straight? I had a vibe in a friend's car that ended up being the angle created by the front mounts not being settled in evenly so the engine was "clocked" slightly off in the chassis, and the output shaft was not pointed exactly where it should have been. It took 2 years for him to realize it. He had tightened the engine mount brackets tight on the engine when it was on the stand, and while you could get the bolts in the K frame it was not sitting level. He found it by looking at the center link in relation to the oil pan with the car's weight on the wheels on level concrete and tires pointed straight ahead.
 
Did you check the universal joints? I had one go bad and I replaced it and smooth going again. Didn't notice if any one specifically mentioned them.
 
Moper- you may be onto something. I did NOT dial-indicate my bellhousing. I thought about it, but I’ve been through that before and it was a lot of trouble for unknown benefit. But I have no chatter at all. Could bellhousing alignment be off so much as to cause pretty significant vibrations in the drivetrain? If it was off that much, wouldn’t I likely have other symptoms? But I have never checked planimetric alignment of the transmission / driveshaft. I’ll do that tomorrow.

I have swapped out both front and rear u-joints; even put on a new slip yoke. Each action changed the vibration ever so slightly but nothing has alleviated the condition.
 
Remove the drive shaft from the pinion yoke and turn it 180 degrees. See if that changes the vibration. Its easy and free to do. Transmission mount?
 
There's a method outlined in the FSM where you put the rear axle on jack stands, remove the rear wheels, and run the car up to speed to catch any vibrations. You can test if the driveshaft is out of balance by attaching a big hose clamp to it and rotating around the screw part on the driveshaft to see if the vibes cancel out or get worse.
 
See post #2.
"What flywheel did you use with it?
Did your machinist balance everything as one unit, balancer, flywheel etc?"
 
You know, if you drive her up to 65/70, put the trans in neutral, and shut off the engine, then if the vibration is gone, it almost had to have been in the engine or trans.But if the vibration remains, it has to be from the mainshaft to the pavement.
So the next thing to do is ride the brakes, in order to drag the pinion down and worsen the the angle. If the vibration gets worse, then the pinion angle or U-joints MAY be the root of it. Or perhaps you just introduced a new vibration that was not there before. But if braking reduces the vibration, then the pinion angle would be suspect.
I have used the car-on--jackstands-with-wheels-on-and-off, several times. You can watch for; wheel hop (Tires out-of-round),wheel imbalance,rim and tire runout,oval drums,a bent axle,and pinion jacking.
 
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